A. Vidović, S. Steiner, R. Škurla
THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
POSSIBILITY OF IMPLEMENTING
NON-CONVENTIONAL AVIATION IN CROATIA
Andrija Vidović, B.Eng.; Sanja Steiner, D.Sc.; Ružica Škurla, B.Eng.
University of Zagreb
Faculty of Transport and Traffic Engineering
Vukelićeva 4, Zagreb, Croatia
avidovic@fpz.hr, ssteiner@fpz.hr, skurla@fpz.hr
ABSTRACT
It is precisely the specific geo-traffic structure of Croatia that results in the need to
implement non-conventional aviation. Improved connections between islands and the
mainland would certainly insure their economic and demographic revitalisation. Conventional
means of air traffic require substantial investments in the infrastructure and are
environmentally unacceptable in places where tourism is to be developed free of pollution.
The construction, however, of the infrastructure facilities needed by the non-conventional
aviation, requires far less investing, being at the same time also more environmentally
friendly. Apart from the commercial purpose, the non-conventional aviation fleet can be used
for the search and rescue services, as well as in providing help in emergency medical
situations.
The fleet of the national flag-carrier, Croatia Airlines, does not meet the requirements
of the regional aviation. The demand for air transport within regions is not high, but it does
exist and therefore a transport means should be offered that would satisfy the needs of the
population efficiently and at an acceptable price. The paper will analyse the parameters of the
non-conventional air transport means that would satisfy the specific needs of the Republic of
Croatia, in particular of the Adriatic coastal region.
1
INTRODUCTION
With her central geotraffic position Croatia represents an important transit area and the
shortest connection between the Western European countries and the Near East. The
development of air traffic in Croatia has prospects since the existing infrastructure represents
the basis and network of air routes at a satisfactory level. However, considering the network
of regional connections within Croatia which, although covering a small area, features a very
unfavourable shape of the borders and a great number of islands, there is need to choose
optimal transportation means that would connect all parts of Croatia in a fast and high-quality
manner. This refers primarily to the islands that are very poorly connected with the mainland.
The logical solution would be the use of non-conventional aviation with the aim of
improving the connections on the whole Croatian territory. However, the question arises
regarding the means of non-conventional aviation whose performances and technical and
technological characteristics would optimally meet the specific needs of the whole Croatian
territory.
ICTS 2003, Nova Gorica, 06-08. November 2003
A. Vidović, S. Steiner, R. Škurla
THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
2
GENERAL FEATURES OF NON-CONVENTIONAL AVIATION
2.1
Introduction
Although the use of conventional means of air traffic still prevails in the world
(CTOL)1, and they require a runway for take-off acceleration, i.e. landing deceleration and
stopping, increasing attention is paid to non-conventional means of air transport which reduce
by their features the required length of the runway, or completely eliminate it. Such aircraft
are of the following types: gyrocopter and helicopter, for vertical takeoff and landing by
vertical thrust, and aircraft with changeable direction of thrust. The essential difference
between conventional and non-conventional aircraft from the technical and technological
aspect refers to the method of realising lift which maintains the aircraft in the air, and the
solution for steering and stabilising the aircraft in flight.
The first step in reducing the need for the length of the runway has been made by the
design of the RTOL2 and STOL3 family of aircraft. The later generation of aircraft that have
shortened the runway even further is the V/STOL4 group of aircraft, and complete elimination
of runways was made possible by the use of non-conventional VTOL5 aircraft.
The technical and technological development of aviation depends on the development
of the power plant. The first attempt at combining the technical and technological
characteristics of CTOL and VTOL aircraft was the "gyrocopter", which is the predecessor of
RTOL aircraft and regarding design, unlike the CTOL aircraft, it has a rotor, whereas unlike
the helicopter as representative of VTOL aircraft, it has the horizontal and the vertical
stabiliser. There are designs without and with the clutch (may be propelled even in flight by
rotor engine) and the designs with shortened wing or without wings.
Further development of aerodynamic systems to increase lift has resulted in the
appearance of STOL aircraft. STOL aircraft have originated by increasing the useful wing
area of CTOL aircraft, by modernising flaps and slots, which resulted in the increase of
aerodynamic lift. The result was the possibility to reduce the approach and takeoff speed thus
allowing for shorter runways. The development of turbo-prop and turbo-jet engines was the
basis for developing the concept of VTOL aircraft.
2.2
Classification of VTOL aircraft
The origin of vertical thrust in VTOL aircraft can be the rotor, propeller, fan propeller,
jet exhaust… Vertical thrust can be generated indirectly so that the engine propels the rotor of
any type, and this group includes gyroplanes, helicopters and convertiplanes. Another way of
generating thrust is directly, when the vertical thrust is obtained by means of the jet from a
turbo-jet or turbo-fan engine. This group includes aircraft with combined power plants in
which one part of the power plant is used to generate vertical, and the other part for
generating horizontal thrust and the aircraft with a single power plant in which both vertical
and horizontal thrust are generated by the same power plant. Another classification of VTOL
aircraft may be made according to the way in which they pass from vertical into level flight.
There are aircraft which tilt the whole aircraft (helicopters), aircraft that tilt the rotor,
1
CTOL - Conventional Take-Off and Landing
RTOL - Reduced Take-Off and landing
3
STOL - Short Take-Off and Landing
4
V/STOL - Vertical/Short Take-Off and Landing
5
VTOL - Vertical Take-Off and Landing
2
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A. Vidović, S. Steiner, R. Škurla
THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
propeller or some other source of thrust (convertiplanes), aircraft which direct thrust by means
of a deflector or by tilting the nozzles (e.g. Harrier) and aircraft with double power plants
which have special engines for taking off and landing vertically, and the engines for level
flight.
2.2.1 Gyroplane
The distinction is between Autogyro – RTOL aircraft and Autogyroplane – V/STOL
aircraft. RTOL requires a run before vertical takeoff. In flight the main rotor rotates by
autorotation which is maintained by progressive speed generated by the engine and most
frequently a traction propeller. The clutch and shaft in Gyroplane allow propelling the rotor,
which is used for takeoff, and results in eliminating the necessary run – V/STOL. After
takeoff, the clutch separates the rotor and the whole engine power is used for the traction
force while rotor rotates by autorotation. If the wings are fitted on the gyroplane, this removes
substantial load off the main rotor which serves to generate lift.
Autogyro is more stable than the helicopter along the longitudinal axis. However, in
spite of certain advantages, the autogyro has more disadvantages such as poor durability, low
maximum takeoff weight and low maximum speed.
Figure 1: Gyroplane-autogyro
Source: http://www.avnet.co.uk/melmorrisjones/gyro1.htm
2.2.2 Vertiplanes
They can reach relatively high cruising speed with VTOL characteristics, and the
design is simple and compact. However, there is a greater number of drawbacks, which is
probably the reason why they never reached serial production:
 relatively low loading capacity and increased specific fuel consumption,
 smaller operation radius (due to greater specific fuel consumption),
 demanding piloting technique while landing (lying on the back and looking over the
shoulder, the pilot has to estimate the position, altitude and landing speed of the
aircraft).
ICTS 2003, Nova Gorica, 06-08. November 2003
A. Vidović, S. Steiner, R. Škurla
THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
Figure 2: Vertiplane
Source: http://www.centennialofflight.gov/essay/Dictionary/VSTOL_tech/DI96G3.htm
2.2.3
Helicopter
Helicopter is the oldest VTOL aircraft concept. Helicopter can be defined as a flying
machine which uses rotating wings (rotor with blades that rotate around the shaft) to generate
lift, propulsion and control of forces allowing the aircraft to hover above ground without the
need for progressive flight in order to realise these forces. Naturally, in order to be practical
and usable, the aircraft must have the capability of flying forward, upwards, cruise at a certain
speed, and decrease altitude again, and hover for landing.
Lift is realised by means of rotor blades. As they rotate, the blades cut the air and
generate lift. Each rotor blade generates the same amount of lift. The rotation of the rotor
around the shaft generates lift which allows helicopter vertical takeoff and landing, as well as
hovering. Tilting of the rotor, which rotates, will cause the flight to proceed in the direction of
the gradient. Such tilting of the rotor causes change in lift from the purely vertical into the
combination of vertical and horizontal lift.
The tail rotor is very important. If the engine for rotation of the rotor is used, the rotor
will rotate, but the engine and the helicopter will tend to turn in the opposite direction. This
phenomenon is called the torque. The tail rotor is used as a small propeller which opposes the
torque and maintains the helicopter level.
The tail rotor is usually connected with the main rotor by means of axes and reductor
system, so that both rotors are mainly connected to the same transmission system, which
means that if the main rotor is turned manually, the tail rotor will turn as well.
By increasing or decreasing the angle of tail rotor blades, the rotor can be used for
steering the helicopter left or right, like a rudder. The tail rotor is connected with the main
rotor via reductor.
ICTS 2003, Nova Gorica, 06-08. November 2003
A. Vidović, S. Steiner, R. Škurla
THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
The latest concept of a helicopter without the tail rotor – NOTAR6, as compensation
for the torque, uses the reactive force of the air directed jet. Of all the VTOL aircraft the
helicopter is the most versatile one, since it can fly both rearwards and sideways, and can
hover at a certain place for a longer period of time. In case of other VTOL aircraft, hovering
is a transition flight regime and it is applied only at takeoff and landing. Unlike e.g.
gyroplane, for the helicopter the autorotation is not a normal performance and is used only in
case of engine failure. The specific feature (and the versatility) of the helicopter lies in the fact
that its main rotor, apart from lift also generates traction force, and also performs the function
of the elevator and control of the angle, thus insuring stability. Therefore, the helicopter rotor
is one of the most complex assemblies.
Drawbacks of a helicopter:
 high level of mechanical complexity,
 increased requirements in maintenance resulting in high exploitation costs,
 high level of vibrations and noise,
 disproportional relation between flight range and loading capacity,
 relatively low cruising speed.
2.2.4 Convertiplane
With all the advantages of vertical takeoff and landing, by watching the helicopter in
progressive flight it is possible to notice its greatest weaknesses, very high profile drag which
influences the unfavourable total lift-drag relation, occurrence of asymmetric lift, air
compressibility, etc., which limit the maximum speed of a helicopter.
The need to have an aircraft that can successfully combine the advantages of vertical
takeoff and landing (VTOL) of helicopters, with high cruising speed, flight range, loading
capacity and economical feature of fixed-wing aircraft has brought to the development of the
tilt-wing and tilt-rotor concepts.
Convertiplane successfully combines the characteristics of the helicopter and the
plane, i.e. it takes off like a helicopter and flies like a plane (HATOL7). This is achieved by
the double use of the rotor. In the takeoff, hovering and landing phase they function as the
supporting helicopter rotor, and in level flight they realise the necessary traction force,
whereas the lift is realised by wings. Such exploitation of the rotor is achieved in two
different concepts:
 by tilting the wing together with the engines – TILT-WING concept (applied on LTV
XC-142A, Canadair-84…),
 by tilting the very rotors i.e. motors– TILT-ROTOR concept (V-22 Osprey, BellBoeing 609…)
6
7
NOTAR-No Tail Rotor
HATOL – Horizontal Attitude TakeOff and Landing
ICTS 2003, Nova Gorica, 06-08. November 2003
A. Vidović, S. Steiner, R. Škurla
THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
Figure 3: LTV XC-142A
Source: http://www.vought.com/heritage/photo/html/pxc-142m_0.html
Figure 4: V-22 Osprey
Source: http://www.military.cz/usa/air/in_service/aircraft/v22a/v22a_en.htm
The tilt-wing concept is a version of the tilt-rotor concept, where both the wings and
engines on them are tilted, which is from the design point of view a much more complex
solution, and it also has aerodynamically negative effects in changing into horizontal flight.
This concept has been abandoned over time.
The tilt-rotor aircraft take off and land vertically with rotors (engines) directed
vertically upwards (like the helicopter rotor). For progressive flight, the rotors located at the
wingtips tilt slowly forward and thus convert the aircraft into a turbo-prop aeroplane. In this
mode of flight, the tilt-rotor can reach much higher flying speed than would be possible by
helicopter. In this way the tilt-rotor combines the advantages of the helicopter and the turboprop aeroplane. Since the rotors of the tilt-rotor cannot be as big as those in helicopters, the
hovering efficiency of tilt-rotor is somewhat lower.
The only disadvantage of convertiplane is reflected in the relatively high level of
mechanical complexity and consequently greater requirements regarding maintenance. Only
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A. Vidović, S. Steiner, R. Škurla
THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
one convertiplane started to be serially manufactured (the HATOL tilt-rotor concept
representative), Bell-Boeing V-22 Osprey. Soon, a civil version of Bell-Boeing 609 is to be
launched on the market with 9 passenger seats, and a version with 20 passenger seats is under
way.
Figure 5: Bell-Boeing 609
Source: http://www.gaservingamerica.org/library_pdfs/BELL_A_1.PDF
3
FEATURES OF NON-CONVENTIONAL AIRCRAFT APPLICABLE IN
CROATIA
Covering an land area of 56,538 km2 Croatia is not a big country, but nevertheless the
shape of its territory is very unfavourable and it has a great number of islands (more than
1100) longitudinally arranged and on the average at a distance of 40 km from the mainland.
By analysing the traffic connections of the islands and the mainland, it may be
concluded that the majority of islands suffer from traffic isolation resulting in even greater
migration of the population. One of the preconditions of demographic revitalisation of the
islands includes better traffic connections between the island and the mainland. Good traffic
connections substantially contribute to economic development of towns and regions and
significantly improve the standard of living. The importance of a well organised traffic system
is best reflected in the data on the share of traffic in the national product of the European
Union of over 6.5 percent and the employment of more than 6 million people. The share of
traffic in the total investments of the companies is estimated at 10-20 percent.
The fleet development in national transport should be oriented to intensifying small
and mid-size aviation which would justify the establishment of scheduled lines between the
Adriatic airports. The development of airports will depend to a great extent on the
development of the regional «commuter» aviation. Within the physical planning strategy the
plans include possible locations for smaller aerodromes on the islands of Hvar, Korčula, Vis,
Mljet, Lastovo, Rab, Pag, Dugi otok and on the mainland in Imotski, Karlovac and Slavonsk
Brod. Naturally, a feasibility study needs to be carried out and the justification for the
construction of so many aerodromes needs to be studied.
Inevitably, the development of traffic is accompanied also by unfavourable ecological
effects such as air and environmental pollution, disturbing the beauty of the landscape, noise
pollution, etc. The construction of smaller aerodromes serving the operation of light aircraft
(reference code 2B), requires an area of 100-200 ha, which includes the runway 700-1200 m
long and 18-30 m wide, apron covering at least 3000 m2 and other supporting facilities
necessary for seamless and safe air traffic operations.
ICTS 2003, Nova Gorica, 06-08. November 2003
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THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
However, even such minimal infrastructure substantially disturbs the beauty of the
landscape, especially on the physically limited areas such as islands.
The implementation of conventional means of air traffic is limited by the economic
aspect of investing in the infrastructure and by the ecological aspect of environmental
protection. Based on the statistical data about the tourist traffic on the islands and the share of
air traffic in the transport of tourists, it can be estimated that the traffic demand does not allow
profitability of the operation of aerodromes on the islands, with the exception of Hvar. Very
significant here is the selection of the optimal location of aerodromes from the aspect of the
gravitation zone. This leads to the conclusion that the selection of the means of air traffic
should be found in the non-conventional aviation, i.e. in using the VTOL aircraft.
From the aspect of traffic demand on the Adriatic islands, the optimal means of air
traffic should be in the «commuter» category, i.e. transport means for intercity traffic
featuring the following characteristics:
 capability of takeoff and landing with minimal length of the runway,
 accommodating 20 passengers with minimal capacity of 2300 kg,
 flight range within 500-1000 km,
 cruising speed of about 500 km/h,
 minimum level of noise, vibrations and exhaust gases,
 good performances at higher summer temperatures and wind of up to 30 m/s,
 economical exploitation.
Argumentation in favour of implementing non-conventional aviation in Croatia results
from its specific geographic and demographic structure. The development of tourism which
means at the same time the economic activities depends on the good solution of the traffic
connections of all parts of Croatia. The priority lies in better traffic connections between the
islands and the mainland and the selection of the traffic branch that would perform the
transport is limited to sea and air traffic.
The comparison analysis of the features provided by helicopters and VTOL aircraft
confirms the justification of using helicopters at distances of up to 100 km, i.e. the use of
VTOL "tilt-rotor" aircraft for distances greater than 100 km. The infrastructure necessary for
takeoff and landing of helicopters and VTOL aircraft is the same, i.e. they require a heliport
area covering 25 X 25 m.
Apart from commercial purposes, VTOL aircraft can be used by the search and rescue
services, as well as for the emergency medical assistance. The basic arguments in favour of
the use of VTOL aircraft and helicopters for the mentioned services are the following:
 existence of only one military helicopter does not satisfy the needs of
emergency aviation,
 regarding the number of traffic accidents and fatalities Croatia is at the very
top according to the European standards,
 unsatisfactory condition of road traffic infrastructure substantially reduces the
efficiency of medical interventions by ambulance vehicles,
 economically, Croatia is oriented towards the development of tourism with
emphasised periodical fluctuations of the population,
 for the tourists the most attractive Adriatic coastal area and the islands are
inaccessible or poorly connected by conventional transport modes, and
therefore usually suffer from traffic isolation,
 due to climatic and meteorological conditions in winter the mountainous
regions of Croatia are mostly cut off regarding traffic.
Analysing only the coast and the islands of the Republic of Croatia, optimal concept of
aircraft would be hydroplane, amphibian. This type of aircraft satisfies all the set
ICTS 2003, Nova Gorica, 06-08. November 2003
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THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
requirements and criteria of the region. This type of aircraft does not require construction of
aerodromes on islands and allows connections of the islands with the continental part of
Croatia. Hydroplane amphibian is a concept of aircraft that features CTOL aircraft
characteristics, and differs regarding undercarriage which allows landing and taking off both
from land and from water (e.g. Canadair). It follows that this type of aircraft would be an
optimal solution for the Croatian islands since it does not require any "fixed" infrastructure on
land because it uses water surface for taking off and landing. By fitting the floats the standard
CTOL aircraft can be converted into a hydroplane. However, the drawback of this type of
aircraft is that it can be used only for landing and takeoff from water surfaces, which makes it
inadequate for landing on the ground. In the world, the use and manufacture of classical
hydroplanes is stagnating, even declining. Only the production of the Canadair aircraft,
mainly used for fire-fighting operations is increasing.
At present, there are no hydroplanes amphibians available on the market that should
according to the mentioned characteristics optimally satisfy the needs for accommodation of
passengers and the capacity.
The comparison analysis of the mid-range parameters (about 500 km) of the CTOL
and hydroplanes shows that hydroplane has up to 100 percent higher costs than CTOL aircraft
and 50 percent lower costs than helicopter.
Compared to CTOL aircraft, the hydroplane amphibian shows the following
advantages:
 no runway is required,
 hydroplanes take off from and land on the water surface, so that the negative
impact of noise and environmental pollution are much reduced,
 they are more attractive for the development of tourism since they allow
landing in almost any bay and are more suitable for excursions,
 increased operative costs are partly reduced since there are no high costs
regarding depreciation of the runway.
The disadvantages of hydroplanes amphibians compared to CTOL aircraft:
 higher operative costs,
 with the increase in nautical tourism it will be difficult to insure the necessary
water surface for landing,
 more than 50 percent of the settlements on the islands are not located on the
coast, so that there is no possibility of direct air transport at places without
aerodromes.
The optimal solution for complete coverage of the whole Croatian territory is an
aircraft designed as a combination of helicopter and plane. Such an aircraft would use the
characteristics of the helicopter for vertical takeoff and landing, and the characteristics of a
plane for the cruising phase. Of course, this would be a non-conventional VTOL aircraft. The
idea of combining the basic concepts of planes and helicopters is about 50 years old. The
technology has found greatest application in the airforce with the aircraft Harrier, and one can
also mention the military aircraft V-22 Osprey with 24 seats or 12 stretchers. A significant
role in the development of VTOL aviation was played by the model XV-15 which flew for the
first time in 1977. Based on that aircraft, a military aircraft V-22 Osprey was developed in
1997 and it has justified to the present day the introduction of the tilt-rotor technology. In the
world there are several types of VTOL aircraft used for commercial purposes. The first nonconventional aircraft for commercial purposes was manufactured by the Bell-Boeing
company with the model 609. Unfortunately, with a capacity of only 9 seats it is obvious that
this type of aircraft does not meet the regional demands of Croatia. The production of model
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THE POSSIBILITY OF IMPLEMENTING NON-CONVENTIONAL AVIATION IN CROATIA
620 is planned, and it could accommodate 20 passengers. However, the testing of this model
will take another number of years and will certainly be expensive, so that the question is, how
real it is to base the planning of the traffic connections among regions using this concept of
aircraft.
4
CONCLUSION
The work presents comparative analysis of the various concepts of aircraft in an
attempt to determine which aircraft concept maximally satisfies the establishment of
connections on the whole territory of the Republic of Croatia at minimum operative costs.
Hydroplane-amphibian has proven as a partial solution in connecting the mainland and the
islands, but considering the fact that almost half of the island settlements are not by the sea,
the technology of door-to-door transport cannot be realised with this concept of aircraft. The
greatest advantage of the hydroplane-amphibian is the fact that this concept does not require
building of aerodromes, but the fact that it only provides connection between the mainland
and the island leads to the conclusion that this type of aircraft is not an optimal solution for
connecting of the whole Croatian territory, but is rather applicable as an additional option of
establishing better connections of the tourist attractive coastal area. Further analysis has led to
the conclusion that the concept of non-conventional tilt-rotor VTOL aircraft represents
optimal solution in connecting the entire area. Regarding infrastructure, the VTOL aircraft
requires only a heliport of dimensions 25x25 m. The available technology in the production of
aircraft and the internationally adopted standards allow realisation and operative use of the
VTOL aircraft in order to meet the needs of establishing regional connections of the whole
Croatian territory, satisfying the requirements of safe, reliable, ecologically acceptable,
economical and door-to-door modern air transport. VTOL concept with its performances
regarding flight, durability, speed, flying altitude, and capacity meets all the elements needed
for operative exploitation in the Republic of Croatia. By unifying the characteristics of
conventional aircraft and helicopter, considering that VTOL aircraft takes off and lands as a
helicopter, and flies on the principle of turbo-prop aircraft almost 95 percent of the flight
regime, a concept of aircraft has been obtained with technical and technological
characteristics which correspond to all the required demands of an optimal aircraft.
REFERENCES
B. Galović, "Prilog razvoju nekonvencionalnih zrakoplova za priobalje Hrvatske",
Doctoral dissertation, Zagreb, 1998.
S. Steiner, Elementi sigurnosti zračnog prometa, Fakultet prometnih znanosti, Zagreb,
1998, pp. 147-152.
J. Božičević, S. Steiner, B. Galović, S. Pavlin M. Tatalović, S. Kaštela, D. Prebežac, R.
Škurla, "Koncepcija strategije razvoja zračnog prometa u RH", Institut prometa i veza
Zagreb, Zagreb, 2002, pp. 57-58.
R. Horonjeff, F. X. McKelvey, Planning & design of airports, McGraw-Hill, Inc., New
York, 1993, pp. 519-530.
http://www.avnet.co.uk/melmorrisjones/gyro1.htm
http://www.centennialofflight.gov/essay/Dictionary/VSTOL_tech/DI96G3.htm
http://www.vought.com/heritage/photo/html/pxc-142m_0.html
http://www.military.cz/usa/air/in_service/aircraft/v22a/v22a_en.htm
http://www.gaservingamerica.org/library_pdfs/BELL_A_1.PDF
http://pollux.com/aeroweb/1996/oct96/tiltrtr.htm
ICTS 2003, Nova Gorica, 06-08. November 2003